DESCRIPTION: Autonomous parvoviruses are small, single-stranded DNA viruses which are incapable of inducing resting cells to enter S-phase and thus replicate exclusively in dividing cell populations. They show complex target cell specificities and can interfere with both normal tissue turnover and the development of neoplastic disease in their adult host. This proposal will focus, in part, on the structural features of the virion which the murine parvovirus MVM uses to initiate infection. Serologic, biochemical and genetic approaches, directed by the crystal structure of the virion, will be combined to unravel the processes by which the virion gains access to the host cell and takes over its macromolecular machinery. In vitro systems will be developed which recapitulate the virus' ability to cross the endosomal membrane, traffic within the cytosol, become uncoated and convert its genome to the duplex form which acts as the initial viral transcription template. All of these processes occur without the assistance of any viral functions other than the signals embedded in its coat. Among the many factors required by the virus once it begins active replication, is a newly-discovered heterodimeric host transcription factor, PIF/GMEB, which is required for the initiation of viral DNA replication, and binds in a novel fashion to a site shared by the initiating viral promoter and the left-end origin of viral DNA replication. The possibility that these interactions underlie the inherent oncotropic behavior of parvoviruses will be tested by determining whether PIF/GMEB activity is functionally altered in transformed compared to normal cells. The knowledge gained in these studies will be applied to the design of viral vectors designed to augment immune surveillance in neoplastic disease of the host, specifically a capsid-replacement vector, based on MVM, which expresses the murine costimulatory T-cell ligand B7-1, and shows significant efficacy in suppressing tumor formation in C57BL/6 mice by the EL4 lymphoma.